Field of the Invention
The invention is related to method for fabricating a semiconductor device, and particularly related to a method for modulating work function of a semiconductor device having a metal gate structure.
Description of Related Art
As the size of a semiconductor device is getting smaller, the size of a gate structure and the thickness of a gate dielectric layer have been reduced. However, leakage current occurs under the condition where the thickness of the gate dielectric layer formed of silicon oxide is reduced. To avoid occurrence of leakage current, the conventional means adopts high dielectric constant (high-k) material to replace silicon oxide to form the gate dielectric layer. Under the condition where high dielectric constant material is used for making the gate dielectric layer, the gate formed of polysilicon reacts with the high dielectric constant material, and Fermi-level pinning is generated consequently. Thus, threshold voltage increases, and the performance of the device is affected accordingly. Therefore, in most cases, a metal gate is used to replace the polysilicon gate.
Currently, it is a common means to adjust the threshold voltage of the semiconductor device by modulating the thickness of work function metal layer or via ion implant tuning. Since it costs more to use ion implant tuning for adjustment, it is more common to adjust the threshold voltage of the semiconductor device by modulating the thickness of work function metal layer.
However, under the condition where the work function of the semiconductor device is modulated through the means of adjusting the thickness of the work function metal layer, if the thickness of the work function metal layer is thin, it is difficult to control process stability and form a continuous film.